Extending the coordination capabilities of tertiary phosphines and arsines: preparation, molecular structure, and reactivity of dinuclear rhodium complexes with PR3 and AsR3 in a doubly bridging coordination mode.

The reactions of [Rh2(kappa2-acac)2(mu-CPh2)2(mu-PR3)] (PR3= PMe34, PMe2Ph 7, PEt38) with an equimolar amount of Me3SiX (X = Cl, Br, I) afforded the unsymmetrical complexes [Rh2X(kappa2-acac)(mu-CPh2)2(mu-PR3)]5, 9-12, which contain the phosphine in a semi-bridging coordination mode. From 4 and excess Me3SiCl, the tetranuclear complex [[Rh2Cl(mu-Cl)(mu-CPh2)2(mu-PMe3)]2]6 was obtained. In contrast, the reaction of 4 with an excess of Me3SiX (X = Br, I) yielded the dinuclear complexes [Rh2X2(mu-CPh2)2(mu-PMe3)]13, 14 in which, as shown by the X-ray crystal structure analysis of 14, the bridging phosphine is coordinated in a truly symmetrical bonding mode. While related compounds with PEt3 and PMe2Ph as bridging ligands were prepared on a similar route, the complex [Rh2Cl2(mu-CPh2)2(mu-PiPr3)]19 was obtained from the mixed-valence species [(PiPr3)Rh(mu-CPh2)2Rh(kappa2-acac)2]17 and HCl. The reaction of [Rh2(kappa2-acac)2(mu-CPh2)2(mu-SbiPr3)]3 with AsMe3 gave the related Rh(mu-AsMe3)Rh compound 21. With Me3SiCl, the acac ligands of 21 can be replaced stepwise by chloride to give [Rh2Cl(kappa2-acac)(mu-CPh2)2(mu-AsMe3)]23 and [[Rh2Cl(mu-Cl)(mu-CPh2)2(mu-AsMe3)]2]24, the latter being isomorphous to the phosphine-bridged dimer 6.